Bound a consumer's identity by the provision it requires (hq issue 263)

The one global 20-character bound made every consumer pay an object
store's key length, even for provisions that keep no name, and a single
overflow refused the provider's whole declaration. An offer now states
its own bound (identity: {max, in} or false); unsaid, a provider told its
consumers keeps 20 and one told nothing keeps none. module check judges
every identity on the longest machine name before merge, and a provider
leaves an overflowing consumer out of its grants and composes, with the
consumer named by push, plan and status (ADR 0225).

cmd/mesh-controller/sendable.go

@@ -43,6 +43,9 @@ type sendable struct {
 	LeftOut []string
 	// leftOutWhy is why each was, for push and plan to say; never on the wire.
 	leftOutWhy map[string]string
+	// withheld is every consumer this machine's grants leave out, because its identity overflows the
+	// provision's bound (novox/hq ADR 0225); for push and plan to say, never on the wire.
+	withheld []catalogue.Overflow
 	// Builds is the build of each module this declaration carries — module to the commit its build
 	// was made from — recorded with the send and never on the wire (novox/hq issue 259, ADR 0221).
 	// Composed only on the send path; nil records that it is not known.